The Effect of Polymer Chain Alignment and Relaxation on Force- Induced Chemical Reactions in an Elastomer

Simultaneous measurements of mechanical response, optical birefringence, and fluorescence signal are acquired in situ during tensile testing of a mechanophore-linked elastomeric polymer. Mechanical stress, deformation, and polymer chain alignment are correlated with force-induced chemical reaction of the mechanophore. The mechanochemically responsive polymer under investigation is spiropyran- (SP-) linked poly(methyl acrylate) (PMA). Force-driven conversion (activation) of SP to its merocyanine (MC) form is indicated by the emergence of a fluorescence signal with 532 nm light incident on the sample. Increasing rate of tensile deformation leads to an increase in both stress and SP-to-MC conversion, indicating a positive correlation between macroscopic stress and activation. Simultaneously collected birefringence measurements reveal that rapid mechanophore activation occurs when maximum polymer chain alignment is reached. It is found that SP-to-MC conversion in PMA requires both a sufficient level of stress and adequate orientation of the polymer chains in the direction of applied force.